1. Field of the Invention
The invention relates to a semiconductor laser driving apparatus for driving a semiconductor laser for modulating an electric signal into a photosignal based on a laser beam and outputting the modulated photosignal.
2. Description of the Related Arts
Hitherto, at the time of a mastering process of a CD (Compact Disc) or a DVD (Digital Versatile Disc), a master disc has been cut by a gas laser using an argon gas or a krypton gas.
FIG. 4 schematically shows a construction of an example of a cutting apparatus using the gas laser. In the cutting apparatus, a photoresist (not shown) coated on a glass substrate 212 is exposed, a latent image is formed on the photoresist, and the cutting is performed. A light source 200 is a gas laser. For example, the gas laser for emitting a laser beam of a short wavelength such as Kr laser for emitting a laser beam having a wavelength λ of 351 nm or an He—Cd laser for emitting a laser beam having a wavelength λ of 442 nm is preferable.
The laser beam emitted from the light source 200 is first converted into the laser beam having predetermined light intensity by an EOM (Electro Optical Modulator) 201 which is driven by a signal electric field that is applied from an APC (Auto Power Controller) 205 and, thereafter, the laser beam is inputted to an analyzer 202. The analyzer 202 is an analyzer for transmitting only an S polarization. The laser beam which transmitted the analyzer 202 becomes the S polarization.
The laser beam emitted from the analyzer 202 transmits a beam splitter 203 and is inputted to a photodetector (PD) 204. In the photodetector (PD) 204, the light intensity of the incident laser beam is detected and a signal corresponding to the light intensity is supplied from the photodetector 204 to the APC 205. The APC 205 adjusts the signal electric field which is applied to the EOM 201 so that the light intensity which is detected by the photodetector 204 becomes constant at a predetermined level. Thus, feedback control is made so that the light intensity of the laser beam which is emitted from the EOM 201 becomes constant.
The laser beam emitted from the light source 200 is reflected by the beam splitter 203 and is inputted to a modulation optical system 206. In the modulation optical system 206, for example, a beam relay optical system comprising, for example, lenses 207 and 209 and an AOM (Acoust Optic Modulator) 208 between them are arranged so as to satisfy a Bragg condition. In the beam relay optical system, they are arranged so that the laser beam emitted from the light source 200 is concentrated on the AOM 208 by using the condenser lens 207.
An EFM (Eight to Fourteen Modulation) signal inputted to a terminal 213 is supplied to a driver 214 and modulated to an ultrasonic wave. The ultrasonic wave is supplied from the driver 214 to the AOM 208. The laser beam inputted to the AOM 208 is modulated on the basis of the ultrasonic wave supplied from the driver 214.
The laser beam modulated in the AOM 208 on the basis of the EFM signal is inputted to an optical system 210 via the lens 209 and converged onto a surface of the disc 212 by an objective lens 211. Thus, the cutting is performed to the disc 212 on the basis of the EFM signal supplied from the terminal 213. In the AOM 208, by bending an optical path of the emitted laser beam by using diffraction of the light, the on/off of the laser beam which is irradiated onto the disc 212 is controlled, thereby performing the cutting.
The intensity or the like of the laser beam which is irradiated onto the surface of the disc 212 needs to be controlled in accordance with a type of disc 212 as a cutting target (for example, whether the disc 212 is a CD or a DVD), a cutting speed, or the like. In the conventional cutting apparatus using the gas laser, a power has uniformly been determined every type of disc and every cutting speed as mentioned above and the cutting is performed. In such conditions, the foregoing on/off control of the laser beam by the AOM 208 has been made.
The gas laser (light source 200) and AOM 208 used in the cutting apparatus according to the prior art have drawbacks such that they are expensive, durability is low, a shape is large, and the like. Therefore, it is demanded to construct the cutting apparatus by a semiconductor laser as a laser device which can solve those drawbacks. In recent years, also in the semiconductor laser, the semiconductor laser which can emit a laser beam of a short wavelength has appeared. By using the semiconductor laser which can emit the laser beam of the short wavelength as a light source, a miniaturized cutting apparatus of low costs can be constructed in place of the cutting apparatus using the gas laser and the AOM.
The semiconductor laser is also used as a light source of light communication besides the foregoing cutting apparatus. The semiconductor laser is driven by a modulation current modulated on the basis of digital data which is transmitted by communication and the emitted laser beam is irradiated, for example, into an optical fiber. The laser beam is received by a photodetector on the light receiving side and an obtained output current is decoded and becomes digital data.
Hitherto, the semiconductor laser is also used in an optical disc recording and reproducing apparatus for recording data onto a recordable optical disc and reproducing the data recorded on the optical disc. A semiconductor laser driving circuit which can be applied to an optical recording and reproducing apparatus for executing the recording and reproduction to/from an optical recording medium has been disclosed in JP-A-63-197037.
A method of adjusting an emission light amount of the semiconductor laser according to the prior art will be schematically explained. Hitherto, the APC and the signal modulating circuit have been assembled in the same loop and one semiconductor laser has been driven. While the APC stabilizes the emission light amount, the signal modulating circuit changes the emission light amount. If those two circuits which execute such opposite operations are assembled in the same loop, for example, when an operating frequency of the APC and a frequency of the modulation signal lie within the same frequency band, the modulation signal is cancelled by the operation of the APC, the emission light amount is set to be constant, and a light modulation signal cannot be obtained.
Therefore, hitherto, for example, by a circuit construction as shown as an example in FIG. 5, an average value of an output from a semiconductor laser 301 is obtained and the APC operation is executed at a frequency which is equal to or lower than hundreds of kHz.
That is, in FIG. 5, the semiconductor laser 301 is driven by a modulation current generated from a modulation current generating circuit 300 on the basis of an EFM signal as a modulation signal. A photodetector 302 receives a laser beam emitted from the semiconductor laser 301 and generates an output current according to the received laser beam. The output current of the photodetector 302 is supplied to an average value detecting circuit 303 and converted into a voltage value. At the same time, for example, it is integrated, an average value is detected, and a bias current is generated from a bias current generating circuit 304 on the basis of the detected average value. The bias current is added to the modulation current for the semiconductor laser, so that the intensity of the laser beam which is outputted from the semiconductor laser 301 is controlled.
In the example of FIG. 5, the APC operation for the semiconductor laser 301 is executed by controlling only the bias current which is applied to the semiconductor laser 301. According to such a method, however, as shown in an example in FIG. 6, there is a problem such that an extinction ratio (a difference between a light level upon light emission and a light level upon light extinction) of the emission light amount changes due to an influence by a temperature change of the semiconductor laser 301 itself. That is, when the temperature of the semiconductor laser 301 itself rises by the continuous driving, the emission light amount of the semiconductor laser 301 decreases. To compensate the decrease in emission light amount, the bias current to the semiconductor laser 301 is increased, so that the light level upon light extinction rises.
There is a problem such that when a duty ratio of the modulation signal changes, the average value of the output current according to the light reception of the laser beam to the photodetector 302 which is detected by the average value detecting circuit 303 changes, so that the emission light amount of the semiconductor laser 301 itself changes.
To solve such a problem, as shown in an example in FIG. 7, there is a method whereby a peak value and a bottom value of a feedback signal based on the output current of the photodetector 302 are sampled and held, respectively, the loop is temporarily shut off, and the APC operation is executed.
That is, in FIG. 7, the output current of the photodetector 302 is converted into a voltage value by an I-V amplifier 310. The voltage value is sampled at regular intervals by a peak value detecting circuit 311 and a bottom value detecting circuit 312 and a peak value and a bottom value are detected, respectively. The bias current is generated from the bias current generating circuit 304 on the basis of the bottom value detected by the bottom value detecting circuit 312. The maximum value of the modulation current is controlled in the modulation current generating circuit 300 on the basis of the peak value detected by the peak value detecting circuit 311.
According to such a method, for example, in light communication or the like using the foregoing semiconductor laser, as shown in an example in FIG. 8A, an interval with the signal and an interval without the signal exist alternately. There is a possibility that when a burst signal is supplied as a modulation signal, a problem occurs. That is, in this case, there is a problem such that when a signal state changes from the non-signal interval to the signal existing interval, it takes a time until the output reaches a predetermined emission output.
For example, at a temperature (room temperature) which is suitable as an operating temperature of the semiconductor laser 301, as shown in FIG. 8C, the laser beam output of a predetermined emission light amount is obtained from the head of the signal existing interval. However, there is a problem such that at a high temperature or a low temperature, since the emission light amount is corrected every sampling intervals with respect to the emission output of the semiconductor laser 301 at such a temperature, it takes a long time until the laser beam output of the proper emission light amount is obtained as shown in FIGS. 8B and 8D.
In recent years, however, a technique for displaying arbitrary characters or pictures onto the recording surface side of the CD or DVD has been proposed. Such a technique uses a principle such that, for example, if a pit which is formed in a certain range on the recording surface is formed in a size smaller than a specified size, when the user observes the recording surface of the disc, such a portion is seen unlike a portion where an ordinary pit has been formed. For example, since the light is easily irregularly reflected by the portion where the pit smaller than the specified size has been formed, such a portion is seen in whitish as compared with the portion where the ordinary pit has been formed. By controlling a pit width on the basis of data constructed so as to display a predetermined character pattern or pictures onto the disc surface, the disc such that the characters or pictures have been displayed on the recording surface side can be formed. The characters or pictures displayed on the recording surface side of the disc are referred to as a “watermark” hereinafter.
If the pit is formed in a size smaller than the specified size, an error rate deteriorates in such a portion upon reproduction. Therefore, for example, correcting means for preventing the deterioration of the error rate by a method whereby a length of portion corresponding to the pit of the recording signal is corrected so that a length of pit is equal to a specified length (actually, it is set to be longer), the length of pit is held at the specified length, and the width is narrowed, or the like is used.
The pit smaller than the specified size can be formed by a method whereby, for example, a power of the laser beam which is irradiated onto the surface of the disc 212 is set to be smaller than that in the case of forming the pit in the specified size at the time of cutting when a master disc is formed.
FIG. 9 schematically shows an example of an emission light amount in the laser beam output at the time of recording the watermark. When the watermark is recorded, control is made in a manner such that the emission light amount of the laser beam output decreases in the portion corresponding to the watermark, that is, in the portion where the pit smaller than the specified size is formed. If the emission light amount is not controlled so as to be constant in the whole area corresponding to the watermark, the display of the watermark fluctuates.
There is a problem such that the control such that a peak light amount of the emission light changes continuously as mentioned above is difficult to be realized by the conventional control method as mentioned above for the semiconductor laser 301.
Hitherto, in the semiconductor laser, the emission light has been modulated by a signal which reciprocates at the maximum and minimum values whose outputs have been set, for example, by a rectangular wave signal and it is difficult to modulate the emission light of the semiconductor laser by the intermediate value of the maximum value and the minimum value.